It has been the norm for electronic systems to operate from a five-volt power supply, thus requiring their electronic components to operate and meet all requirements from a single 5-volt supply. These requirements may include interfacing logic levels and timing specifications, and may apply to other devices such as other internal circuits. Increasingly, electronic systems, particularly analog IC's, are migrating to a 3.3 volt power supply which conforms to the recently approved JEDEC Standard 8-1A. This standard also defines 3.3 volt-compatible logic levels, which are different from those levels using a 5-volt supply. Although it would be desirable to have electronic components which are capable of conforming to either supply standard, redesigning the components and their front-end structures for operations on a 3-volt supply would create delays and uncertainties.
To further complicate the problem, in many system applications, it is not known beforehand whether the supply will be 5-volt or 3.3-volt. An example of this scenario is the PCMCIA-compatible cards, which may be functional at 3.3 volt but plugged into a 5-volt system such as a laptop computer. Detection and adaptation by the PCMCIA card thus become an integral part of the power-on process. Without the procedure prior to power-on, the PCMCIA card may be operating under an incompatible supply and system configuration.
Conventionally, to provide 3.3 volt supply to an integrated analog ("IA") circuit, which operates at a 5 volt level, a voltage regulator and voltage doubler are implemented with the IA circuit. The voltage regulator and voltage doubler generate a DC voltage of 5 volt regardless of the system power supply being 5 volt or 3.3 volt. This approach allows the 5-volt components of a system, such as a PCMCIA card for a notebook computer to be functional with both 5-volt and 3.3-volt supplies without the need to redesign the existing components.
The conventional approach, however, requires the system to generate an accurate reference voltage for the voltage regulator and voltage doubler so that the voltage generated thereafter is as close to 5 volt as possible. Although most IA circuits have an on-chip bandgap reference voltage, this bandgap reference voltage in many cases may only be operational under a 5-volt steady supply. Since the voltage regulator and voltage doubler also require an accurate reference voltage to generate a DC voltage of about 5 volt for invoking the bandgap voltage, the voltage regulator and voltage doubler cannot use the bandgap reference voltage as their reference voltage during system power-up transients. Therefore, it would be desirable to generate an accurate reference voltage, irrespective of the 3.3-volt or 5-volt power supply from the system, for the voltage regulator and voltage doubler to generate a 5-volt DC voltage so as to activate the bandgap reference voltage. It would also be desirable to use the bandgap reference voltage, solely, once the voltage regulator and voltage doubler are stabilized after the power-up transients have settled.
Furthermore, if the power supply from the system is already 5 volts, it would be desirable to take advantage of it directly by subsequently bypassing the voltage regulator and voltage doubler loop. This objective would require a mechanism of detecting whether the system supply is 5 volt or 3.3 volt. Once the detection is completed, the information can help the system reconfigure its own interfaces and other internal circuitry accordingly.
With respect to the voltage regulator and voltage doubler, it is also desirable to have an efficient DC-DC up-converter for converting a system supply voltage for the IA circuit without going above the CMOS processing tolerance.